[PATCH 5/5] xfs: reverse search directory freespace indexes

Dave Chinner
2018-10-25T07:27:26Z

From: Dave Chinner <dchinner@redhat.com>

When a directory is growing rapidly, new blocks tend to get added at
the end of the directory. These end up at the end of the freespace
index, and when the directory gets large finding these new
freespaces gets expensive. The code does a linear search across the
frespace index from the first block in the directory to the last,
hence meaning the newly added space is the last index searched.

Instead, do a reverse order index search, starting from the last
block and index in the freespace index. This makes most lookups for
free space on rapidly growing directories O(1) instead of O(N), but
should not have any impact on random insert workloads because the
average search length is the same regardless of which end of the
array we start at.

The result is a major improvement in large directory grow rates:

create time(sec) / rate (files/s)
File count vanilla patched
10k 0.54 / 18.5k 0.52 / 19.3k
20k 1.10 / 18.1k 1.00 / 20.0k
100k 4.21 / 23.8k 3.58 / 27.9k
200k 9.66 / 20,7k 7.08 / 28.3k
1M 86.61 / 11.5k 38.33 / 26.1k
2M 206.13 / 9.7k 82.20 / 24.3k
10M 2843.57 / 3.5k 591.78 / 16.9k

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
---
fs/xfs/libxfs/xfs_dir2_node.c | 69 +++++++++++++++++------------------
1 file changed, 34 insertions(+), 35 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_dir2_node.c b/fs/xfs/libxfs/xfs_dir2_node.c
index 621f63de075c..aa52dc740305 100644
--- a/fs/xfs/libxfs/xfs_dir2_node.c
+++ b/fs/xfs/libxfs/xfs_dir2_node.c
@@ -1747,7 +1747,8 @@ xfs_dir2_node_find_freeblk(
struct xfs_inode *dp = args->dp;
struct xfs_trans *tp = args->trans;
struct xfs_buf *fbp = NULL;
- int findex;
+ int findex = 0;
+ xfs_dir2_db_t firstfbno;
xfs_dir2_db_t lastfbno;
xfs_dir2_db_t ifbno = -1;
xfs_dir2_db_t dbno = -1;
@@ -1775,7 +1776,7 @@ xfs_dir2_node_find_freeblk(
ASSERT(be16_to_cpu(bests[findex]) != NULLDATAOFF);
ASSERT(be16_to_cpu(bests[findex]) >= length);
dbno = freehdr.firstdb + findex;
- goto out;
+ goto found_block;
}

/*
@@ -1784,9 +1785,10 @@ xfs_dir2_node_find_freeblk(
*/
ifbno = fblk->blkno;
fbno = ifbno;
+ xfs_trans_brelse(tp, fbp);
+ fbp = NULL;
+ fblk->bp = NULL;
}
- ASSERT(dbno == -1);
- findex = 0;

/*
* If we don't have a data block yet, we're going to scan the freespace
@@ -1797,6 +1799,7 @@ xfs_dir2_node_find_freeblk(
if (error)
return error;
lastfbno = xfs_dir2_da_to_db(args->geo, (xfs_dablk_t)fo);
+ firstfbno = xfs_dir2_byte_to_db(args->geo, XFS_DIR2_FREE_OFFSET);

/* If we haven't get a search start block, set it now */
if (fbno == -1)
@@ -1804,51 +1807,47 @@ xfs_dir2_node_find_freeblk(

/*
* While we haven't identified a data block, search the freeblock
- * data for a data block with enough free space in it.
+ * data for a good data block. Do a reverse order search, as growing
+ * directories will put new blocks with free space at the end of the
+ * free space index.
*/
- for ( ; fbno < lastfbno; fbno++) {
- /* If we don't have a freeblock in hand, get the next one. */
- if (fbp == NULL) {
- /* If it's ifbno we already looked at it. */
- if (fbno == ifbno)
- continue;
+ for (fbno = lastfbno - 1; fbno >= firstfbno; fbno--) {
+ /* If it's ifbno we already looked at it. */
+ if (fbno == ifbno)
+ continue;

- /*
- * Read the block. There can be holes in the freespace
- * blocks, so this might not succeed. This should be
- * really rare, so there's no reason to avoid it.
- */
- error = xfs_dir2_free_try_read(tp, dp,
- xfs_dir2_db_to_da(args->geo, fbno),
- &fbp);
- if (error)
- return error;
- if (!fbp)
- continue;
+ /*
+ * Read the block. There can be holes in the freespace
+ * blocks, so this might not succeed. This should be
+ * really rare, so there's no reason to avoid it.
+ */
+ error = xfs_dir2_free_try_read(tp, dp,
+ xfs_dir2_db_to_da(args->geo, fbno),
+ &fbp);
+ if (error)
+ return error;
+ if (!fbp)
+ continue;

- findex = 0;
- free = fbp->b_addr;
- bests = dp->d_ops->free_bests_p(free);
- dp->d_ops->free_hdr_from_disk(&freehdr, free);
- }
+ findex = 0;
+ free = fbp->b_addr;
+ bests = dp->d_ops->free_bests_p(free);
+ dp->d_ops->free_hdr_from_disk(&freehdr, free);

/* Scan the free entry array for a large enough free space. */
- do {
+ for (findex = freehdr.nvalid - 1; findex >= 0; findex--) {
if (be16_to_cpu(bests[findex]) != NULLDATAOFF &&
be16_to_cpu(bests[findex]) >= length) {
dbno = freehdr.firstdb + findex;
- goto out;
+ goto found_block;
}
- } while (++findex < freehdr.nvalid);
+ }

/* Didn't find free space, go on to next free block */
xfs_trans_brelse(tp, fbp);
- fbp = NULL;
- if (fblk)
- fblk->bp = NULL;
}

-out:
+found_block:
*dbnop = dbno;
*fbpp = fbp;
*findexp = findex;
--
2.19.1